In human GWAS SNPs in the tetratricopeptide repeat domain protein 39B gene (TTC39B/T39) that cause reduced hepatic expression are associated with increased HDL cholesterol. T39, a scaffolding protein, promotes the ubiquitination and turnover of LXR. Western Type Diet (WTD)- fed Ldlr-/-T39-/- mice showed decreased fatty liver, increased HDL, decreased LDL and reduced atherosclerosis. T39 deficiency inhibited processing of sterol regulatory element binding protein 1 (SREBP-1) into its nuclear active form, due to an increase in microsomal phospholipids containing polyunsaturated fatty acids (PUFA). We discovered that T39-/- macrophages also show increased LXR., increased expression of genes mediating synthesis of PUFA, and reduced inflammatory responses, suggesting a role of macrophages in reduced atherosclerosis in mice with whole body T39 deficiency. Interestingly, treatment of WTD-fed Ldlr-/- mice with a T39 antisense oligonucleotide (ASO) that decreased expression in both liver and adipose tissue leads to reduced adiposity. While brown adipose tissue depots and food intake were unchanged, T39 ASO treatment led to markedly increased expression of Ucp1 in WAT, suggesting formation of beige adipocytes (beiging). We will investigate the role of T39 in regulating LXR in macrophages and PPAR? in adipose tissue and effects on adiposity, insulin resistance and atherosclerosis (Fig 1), linking to LXR studies in Proj 1, and of PPAR? studies in Proj 3. Aim 1 will assess the impact of myeloid T39 deficiency on macrophage inflammation, insulin resistance and atherosclerosis. We will explore the hypothesis that T39 deficiency reduces ubiquitination and promotes stabilization of LXR in macrophages and thus enhances LXR-mediated repression of inflammatory genes, primarily by increasing the synthesis of phospholipids containing long chain PUFA. With Dr Tabas, we will determine if macrophage T39 deficiency promotes inflammation resolution.To determine if myeloid T39 deficiency is anti- atherogenic, we will breed LysM-CreT39f/f mice in the Ldlr-/- background. Aim 2 will assess the impact of adipocyte T39 deficiency on adiposity, insulin resistance and atherosclerosis. We will breed AdipoQ-CreT39f/f mice in order to deplete T39 in WAT and assess adiposity, beiging and, on the Ldlr-/- background, plasma lipoproteins and atherosclerosis. With Drs Accili and Qiang, we will examine the impact of T39 deficiency on formation of beige adipocytes from the stromal- vascular fraction (SVF). We will assess a specific hypothesis that T39 deficiency decreases the binding of Rb family members to the E2F binding site in the promoter of Pparg1and other beiging genes, promoting formation of beige adipocytes.